Conventionally, there has been known a four-wheel drive vehicle constituted by an FR (front engine rear drive) of a vertically installed engine type as one of four-wheel engine vehicles which can be switched between a two-wheel drive state and a four-wheel drive state. The four-wheel drive vehicle as previously mentioned is generally constructed to include a center differential for absorbing the rotational difference between front wheels and rear wheels while travelling in the four-wheel drive state, and a switching mechanism capable of switching the two-wheel drive state and the four-wheel drive state.
As one of transfer devices of this kind, there has been known a transfer device which comprises a center differential having a deferential case connected with an output shaft of a transmission, one side gear connected with a rear drive shaft, and the other side gear connected with an output member forming part of a front wheel drive system, and a switching mechanism juxtaposed with the center differential (see for example Patent Document 1).
The switching mechanism disclosed in the Patent Document 1 has a sleeve which is spline-engageable with splines respectively formed on the outer peripheral portion of the differential case, on the outer peripheral portion of an output member and on the outer peripheral portion of a drive member capable of transmitting power to a front wheel drive shaft, and which is axially movable. The above sleeves can axially be moved by the operation of a driver, thereby making it possible to selectively switch the engagement states with the splines, and thus can take three positions as follows. The positions include a first position where the sleeve is in the engagement state with the spline of the differential case and the spline of the output member, a second position where the sleeve is in the engagement with the spline of the output member and the spline of the drive member by the movement of the sleeve toward the drive member from the first position, and a third position where the sleeve is in the engagement state with the above three splines by the further movement of the sleeve toward the drive member from the second position.
At the first position, the rotation of the differential case is not transmitted to the drive member, so that the transfer device takes the two-wheel drive state in which the front wheel is not driven while only the rear wheel is driven. At the second position, the rotation of the other side gear is transmitted to the drive member, so that the transfer device takes the four-wheel drive state in which the front wheel and the rear wheel are driven. At this time, the differential case and the output member are not fixed through the sleeve, so that the transfer device takes a differential free state in which the center differential can absorb the rotational difference between the front and rear wheels. At the third position, the rotation of the differential case is transmitted to the drive member, so that the transfer device takes the four-wheel drive state in which the front wheels and the rear wheels are driven. At this time, the differential case is fixed to the output member, so that the power inputted from the transmission is directly transmitted to the drive member through the differential case but not through the other side gear. This means that at the third position, the rotational difference between the front and rear wheels is not absorbed by the center differential, viz., the transfer device takes a differential lock state.